Band Gap in Magnetite above Verwey Temperature Induced by Symmetry Breaking

2017-10-24T00:00:00Z (GMT) by Hongsheng Liu Cristiana Di Valentin
Magnetite exhibits a famous phase transition, called Verwey transition, at the critical temperature <i>T</i><sub>V</sub> of about 120 K. Although numerous efforts have been devoted to the understanding of this interesting transition, up to now, it is still under debate whether a charge ordering and a band gap exist in magnetite above <i>T</i><sub>V</sub>. Here, we systematically investigate the charge ordering and the electronic properties of magnetite in its cubic phase using different methods based on density functional theory: DFT+U and hybrid functionals. Our results show that, upon releasing the symmetry constraint on the density but not on the geometry, charge disproportionation (Fe<sup>2+</sup>/Fe<sup>3+</sup>) is observed, resulting in a band gap of around 0.2 eV at the Fermi level. This implies that the Verwey transition is probably a semiconductor-to-semiconductor transition and that the conductivity mechanism above <i>T</i><sub>V</sub> is small polaron hopping.